The investigation deals with the measurement of fetal and maternal placental capillary blood pressures, osmotic pressures and resistances to flow. It will further investigate the Frank-Starling mechanism of the fetal heart, and the regulation of fetal and maternal arterial blood pressures. Further investigations are designed to demonstrate the existence of fetal regulatory mechanisms, other than the well known cardiovascular mechanisms that occur in the adult also, that interact with the stabilization of fetal placental blood flow, since the ordinary regulation mechanisms can be shown not to be capable of regulating steady state fetal placental blood flow. A method is described for measuring set point and open loop gain of the baroreceptor reflex in unanesthetized animals. The carotid sinus reflex, which appears to be weak in the fetus, will be investigated by this method.